This invention relates to a level indicating system and, in particular, to a level indicating system which is selectively operable to provide an indication of the peak-level of a signal or the volume unit of said signal.
In sound signal processing techniques, such as in recording audio signals or mixing audio signals, it is often desireable to obtain an indication of the peak value of the audio signal and/or an indication of the resultant volume of the sound. Heretofore, such indications have usually required individual indicators, such as a peak-level indicator and a VU indicator. The VU indicator provides an indication of volume units which is a measure of the audio power of a sound signal. The VU indication is in correspondence with the sense of hearing of the reproduced sound. By providing peak-level and VU indications the optimum level of the processed audio signals can be determined and the desired sound processing can be readily achieved.
To provide an indication of the peak-level of a signal, a suitable detector usually requires a relatively rapid rise time to permit a rapid response to the changes of the input signal and a relatively slow decay time to thereby permit a response only to the peak-levels of the input signal. A conventional peak-detecting circuit exhibits the necessary time constant characteristics to provide indications of signal peak-levels. A VU indicator, however, provides an indication of the power levels included in the input signal and thus requires a response characteristic having a relatively slow rise time and a relatively rapid decay time. Conventional filter and time constant circuits having such response characteristics are well known.
As both peak-level indications and VU indications are often quite desirable for the aforenoted reasons, it is advantageous to provide a single device that is readily capable of providing both types of indications. Since the significant differences between a peak-level indicator and VU indicator reside in the differences in rise and decay time characteristics, it might be thought that a combined indicator could be formed by a diode and resistor in series with an amplifier, the resistor having a variable resistance value so as to change the time constant of the circuit. Depending upon the particular resistance value thereof, the time constant of the circuit could exhibit a relatively rapid rise time and a slow decay time, characteristic of a peak-level indicator. The same circuit, after a change in the resistance value thereof, could then exhibit a relatively slow rise time and a rapid decay time, characteristic of a VU indicator. However, it is advantageous to dispose this circuit in a feedback loop for the amplifier to thereby achieve desired circuit operation. An attendant disadvantage with such feedback loop is that, as the resistnce value is changed, the gain of the feedback loop is correspondingly varied. Accordingly, the output signal produced when the circuit is operated as a peak-level indicator would exhibit a value that is unrelated to the output signal produced when the circuit is operated as a VU indicator. This discrepancy would thus provide different indications for the same input signal value and would thus lead to erroneous signal processing.
As an alternative to the aforenoted circuit, a combined peak-level and VU indicator might be formed of a peak-detecting circuit having an output transmitted through a field effect transistor (FET) and then through a single time-constant circuit to an indicating device. In this proposal, the single time-constant circuit can exhibit variable time constant characteristics so that the indicator could be operated with first time constant characteristics having a relatively rapid rise time and a slow decay time, whereby peak-level indications are provided; and the indicator could be operated with second time constant characteristics having a relatively slow rise time and a rapid decay time, whereby VU indications are provided. The particular time constant characteristics can, of course, be determined in accordance with the particular parameters selected for the single time-constant circuit. Although the ultimate output signals produced by such a circuit are, of course, dependent upon the specific circuit design, it is usual for such circuit to provide a VU indication that is approximately 4dB lower than the peak-level indication. Although calibration of the indicator device, such as an indicating meter, might compensate for such divergence in the signal level indications, it is often quite difficult to bring the reference calibration for the peak-level indicator into correspondence with that of the VU indicator.